x/exp/shiny: build is broken on darwin

[rakyll]

$ go build golang.org/x/exp/shiny/...
# golang.org/x/exp/shiny/widget
widget/label.go:40: undefined: font.MeasureString
widget/label.go:41: m.Ascent.Ceil undefined (type fixed.Int26_6 has no field or method Ceil)
widget/label.go:41: m.Descent.Ceil undefined (type fixed.Int26_6 has no field or method Ceil)
widget/label.go:57: m.Ascent.Ceil undefined (type fixed.Int26_6 has no field or method Ceil)

/cc @nigeltao @crawshaw

[nigeltao]

You need to sync your golang.org/x/image repo.